The metaphysical literature underlying Sankara’s expositions consists essentially of the Four Vedas together with the Brahmanas and their Upanishads, all regarded as revealed, eternal, datable (as to their recension, in any case) before 500 B.C., together with the Bhagavad Gita and Brahma Sutra (datable before the beginning of the Christian era). Of these books, [4] the Vedas are liturgical, the Brahmanas are explanatory of the ritual, and the Upanishads are devoted to the Brahma-doctrine or Theologia Mystica, which is taken for granted in the liturgyliturgieλειτουργίαleitourgíaliturgyliturgialiturgía and ritual. The Brahma Sutra is a greatly condensed compendium of Upanishad doctrine, and the Bhaga-vad Gita is an exposition adapted to the understandingVerständniscompreensãoentendimentocompréhensionententecomprensiónunderstanding of those whose primary business has to do with the active rather than the contemplative life.